class Effective::Order

def add(*items, quantity: 1)
  raise 'unable to alter a purchased order' if purchased?
  raise 'unable to alter a declined order' if declined?
  cart_items = items.flatten.flat_map do |item|
    if item.kind_of?(Effective::Cart)
      item.cart_items.to_a
    elsif item.kind_of?(ActsAsPurchasable)
      Effective::CartItem.new(quantity: quantity, purchasable: item)
    elsif item.kind_of?(Effective::Order)
      # Duplicate an existing order
      self.note_to_buyer ||= item.note_to_buyer
      self.note_internal ||= item.note_internal
      self.cc ||= item.cc
      item.order_items.select { |oi| oi.purchasable.kind_of?(Effective::Product) }.map do |oi|
        purchasable = oi.purchasable
        product = Effective::Product.new(name: purchasable.purchasable_name, price: purchasable.price, tax_exempt: purchasable.tax_exempt)
        # Copy over any extended attributes that may have been created
        atts = purchasable.dup.attributes.except('name', 'price', 'tax_exempt', 'purchased_order_id').compact
        atts.each do |k, v|
          next unless product.respond_to?("#{k}=") && product.respond_to?(k)
          product.send("#{k}=", v) if product.send(k).blank?
        end
        Effective::CartItem.new(quantity: oi.quantity, purchasable: product)
      end
    else
      raise 'add() expects one or more acts_as_purchasable objects, or an Effective::Cart'
    end
  end.compact
  # Make sure to reset stored aggregates
  assign_attributes(subtotal: nil, tax_rate: nil, tax: nil, amount_owing: nil, surcharge_percent: nil, surcharge: nil, total: nil)
  retval = cart_items.map do |item|
    order_items.build(
      name: item.name,
      quantity: item.quantity,
      price: item.price,
      tax_exempt: (item.tax_exempt || false),
    ).tap { |order_item| order_item.purchasable = item.purchasable }
  end
  retval.size == 1 ? retval.first : retval
end